The long term objective of this proposal is to obtain a complete description of the role of electrical excitability in the regulation of hormone secretion. Of specific interest is the role of calcium channels in the regulation of prolactin secretion from an anterior pituitary cell line (GH3). The secretion of prolactin from these cells is enhanced by thyrotropin releasing hormone, vasoactive intestinal peptide and estrogen and inhibited by somatostatin and acetylcholine. A variety of electrophysiological techniques (whole cell voltage and current clamp and single channel patch clamp recordings) will be used to analyze the efffects of these substances on membrane electrical properties. The specific aims of the proposal are: 1) To characterize the voltage and Ca++ sensitivity, and pharmacology, of single K+ and Ca++ channel currents in cell attached and excised patches of membrane, 2) To examine the effects of a variety of neurotransmitters and hormones that influence prolactin secretion from GH3 cells on membrane electrical properties and to elucidate the various mediators involved in the signal transduction process, 3) To examine the effects of these receptor agonists on fluorescence measurements of cytosolic free Ca++ levels, and (4) To examine the above properties in primary cultures of prolactin secreting cells from rat. The electrical measurements will help provide an understanding of the mechanisms by which various agonists influence resting membrane potential and the firing rate and shape of Ca++ dependent action potentials in GH3 cells and cultured lactotrophs. This information will be correlated with the analysis of cytosolic Ca++ levels to determine the extent to which regulation of Ca++ entry through membrane Ca++ channels can contribute to agonist mediated regulation of prolactin secretion. Such information may ultimately provide insights for the therapeutic treatment of disorders related to abnormal secretion of prolactin and other anterior pituitary hormones.